Conventionally, methods of this type are applied both in computer systems having distributed resources for solving complex processing tasks and, by way of example, in communication networks having a large number of subscribers and also a central electronic billing system. Put in perspective, they have great economic significance in the development and operation of IN services with charge debiting from a prepaid credit.
In the evolution of an IN service (prepaid service), it is possible to reserve partial sums or portions of a service user's and account holder's (subscriber's) credit. The money used up for a telephone call is normally deducted at the end of the call. Only then are the reservations for the call reclaimed.
When calls have been terminated, it is necessary to ensure that the reservations made for them are released again even without this final action. If not, it would no longer be possible to use up the entire credit for telephoning.
This is currently achieved by checking, when any call accesses the account, how long it has been since this account was last accessed. If a prescribable (recovery) time has been exceeded, all reservations are deleted. Upon resetting, it is necessary to ensure that no “live” call is holding a reservation.
With mobile radio links based on the GPRS standard, the account is accessed for always-on scenarios continuously, however, i.e. at least the (charge) portion of the GPRS call is reserved up to the next ACR at all times. It is therefore never possible to reset the reservations on the basis of the current method. The reservations for parallel calls terminated under some circumstances are no longer available to the subscriber.
The main problem is concealed in the recovery mechanism: the recovery time transferred for the RESERVE does not relate to the individual reservation, but rather all reservations are reset to zero if the transferred time since the last reserving access has expired. This results in “hanging” reservations being cancelled only when there is at least one break of the length of the recovery time between two calls. In the case of “normal” calls, this behavior is not so critical, since                the recovery time can generally be kept relatively short in this case (billing on the basis of time), and        the calls do not last forever, which means that there is a very high chance of a break coming soon.        
With GPRS, there is the problem that the calls can last for any length of time (volume-based billing). “Hanging” reservations hardly have any chance, or sometimes have no chance, of being cancelled.
In the case of lengthy calls (GPRS calls or else other calls) or calls in brief succession, reservations for parallel or previous calls which have been terminated may, for the same reason, be released again too late from the subscriber's point of view.